1. Field of the Invention
The invention relates to communication systems. More particularly, the invention relates to load estimation in a multiple access system.
2. Description of the Related Art
FIG. 1 is an exemplifying embodiment of a terrestrial wireless communication system 10. FIG. 1 shows three remote units 12A, 12B and 12C and two base stations 14. In reality, typical wireless communication systems may have many more remote units and base stations. In FIG. 1, the remote unit 12A is shown as a mobile telephone unit installed in a car. FIG. 1 also shows the portable computer remote unit 12B and the fixed location remote unit 12C such as might be found in a wireless local loop or meter reading system. In the most general embodiment, the remote units may be any type of communication unit. For example, the remote units can be hand-held personal communication system (PCS) units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link signal 18 from the base stations 14 to the remote units 12 and a reverse link signal 20 from the remote units 12 to the base stations 14.
In a typical wireless communication system, such as that illustrated in FIG. 1, some base stations have multiple sectors. A multi-sectored base station comprises multiple independent transmit and receive antennas as well as independent processing circuitry. The principles discussed herein apply equally to each sector of a multi-sectored base station and to a single sectored independent base station. For the remainder of this description, therefore, the term xe2x80x9cbase stationxe2x80x9d can be assumed to refer to either a sector of a multi-sectored base station or a single sectored base station.
In a code division multiple access (CDMA) system, remote units use a common frequency bandwidth for communication with all base stations in the system. Use of a common frequency bandwidth adds flexibility and provides many advantages to the system. For example, use of a common frequency bandwidth enables a remote unit to simultaneously receive communication signals from more than one base station, as well as transmit a single signal for reception by more than one base station. The remote unit discriminates the simultaneously received signals from the various base stations through the use of the spread spectrum CDMA waveform properties. Likewise, the base station can discriminate and separately receive signals from a plurality of remote units.
Various methods exist for transferring communication with the remote unit from one base station to another through a process known as handoff. Handoff may be necessary if a remote unit operating in the coverage area of an original base station moves into the coverage area of a target base station. One method of handoff used in CDMA systems is termed a xe2x80x9csoftxe2x80x9d handoff. Through the use of soft handoff, communication with the target base station is established before termination of communication with the original base station. When the remote unit is communicating with two base stations, both the remote unit and base stations create a single signal from the multiple received signals. Through the use of soft handoff, communication between the remote unit and the end user is uninterrupted by the eventual handoff from the original base station to the target base station. U.S. Pat. No. 5,267,261 entitled xe2x80x9cMOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated herein by this reference, discloses a method and system for providing communication with a remote unit through more than one base station during the handoff process.
In a wireless system, maximizing the capacity of the system in terms of the number of simultaneous calls that can be handled is extremely important. System capacity in a spread spectrum system is increased if the power received at the base station from each remote unit is controlled such that each signal arrives at the base station receiver at the minimum level required to maintain the link. If a signal transmitted by a remote unit arrives at the base station receiver at a power level that is too low, the signal to interference ratio may be too low to permit high quality communication with the remote unit. If, on the other hand, the remote unit signal arrives at a power level that is too high, communication with this particular remote unit is acceptable, but the high power signal acts as unnecessary interference to other remote units. This unnecessary interference may adversely affect communications with other remote units. Thus, in general, a remote unit located near the base station transmits a relatively low signal power while a remote unit located at the edge of the coverage area transmits a relatively high signal power.
In standard CDMA systems, in order to increase capacity, the power transmitted by a remote unit over the reverse link is controlled by each base station through which active communication is established (i.e. each base station with which the remote unit is in soft handoff.) Each base station through which communication is established measures the received signal quality and compares it to a desired set point. Each base station, periodically, generates and sends a power adjustment command to the remote unit. The power adjustment commands puncture the user traffic data on the forward link traffic channel.
The power adjustment command orders the remote unit to either increase or decrease the power at which it is transmitting the reverse link signal. The remote unit increases its transmit power level only if every base station commands an increase. In this way, the transmit signal power of a remote unit in soft handoff is controlled mainly by the base station which receives its signal at the highest signal quality. A system for base station and remote unit power control is disclosed in U.S. Pat. Nos. 5,056,109, 5,265,119, 5,257,283 and 5,267,262 which are assigned to the assignee hereof.
In more advanced systems, in addition to controlling the power level at which the remote unit transmits on the reverse link, the data rate at which the remote unit transmits on the reverse link is also controlled. A remote unit located on the edge of a coverage area can reduce the data rate at which it transmits in order to increase the signal quality of the signal as received at the base station. By reducing the data rate, the time devoted to each bit may be increased, thus, increasing the energy devoted to each bit and increasing the performance of the link. A remote unit which transmits at less than the full rate generates less interference and consumes less system resources than a remote unit transmitting at full rate, thereby, freeing system resources for use by other remote units.
The power adjustment commands compensate for the time-varying path loss in the wireless channel. Path loss in the wireless channel is defined as degradation or loss suffered by a signal as it travels between the remote unit and the base station. Path loss is characterized by two separate phenomenon: average path loss and fading. In a typical wireless system, the forward link and reverse link operate on different frequencies. Nevertheless, because the forward and reverse links operate within the same frequency band, a significant correlation exists between the average path loss of the two links. On the other hand, fading is an independent phenomenon for the forward and reverse link and varies rapidly as a function of time, especially when the remote unit is in motion or is located near objects in motion.
In an exemplifying wireless system, each remote unit estimates the path loss of the forward link based on the total power at the input of the remote unit. The total power is the sum of the power from all base stations operating on the same frequency assignment as perceived by the remote unit. From the estimate of the average forward link path loss, the remote unit sets a nominal transmit power level of the reverse link signal. As noted above, each base station with which the remote unit has established communications sends power adjustment commands to the remote unit to compensate for differences between the path loss on the forward link and the path loss on the reverse link, for fading, and for other sources of error. Thus, the remote unit creates a reverse link transmit gain adjustment signal which increases or decreases the reverse link transmit power level above or below the nominal level determined by estimating the average forward link path loss. The value of the transmit gain adjustment signal is the accumulated effect of the power control adjustment commands received from the base station over a period of time. If the remote unit receives an equal number of turn-up and turn-down commands, the value of the transmit gain adjustment signal is zero and the reverse link transmit level is equal to the nominal value. If the remote unit receives more turn-up than turn-down commands, the value of the transmit gain adjustment signal is logically positive and the reverse link transmit level is greater than the nominal value. If the remote unit receives more turn-down than turn-up commands, the value of the transmit gain adjustment signal is logically negative and the reverse link transmit level is less than the nominal value.
If a minimum acceptable signal quality is specified, an upper bound on the number of simultaneous users which can communicate through a base station can be calculated. This upper bound is commonly referred to as the pole capacity of a system. The ratio of the actual number of users to the pole capacity is defined as the loading of the system. As the number of actual users approaches the pole capacity, loading approaches unity. A loading close to unity implies potentially unstable behavior of the system. Unstable behavior can lead to degraded performance in terms of voice quality, high error rates, failed handoffs, and dropped calls. In addition, as loading approaches unity, the size of the coverage area of the base station shrinks such that users on the outer edge of the no-load coverage area are no longer be able to transmit sufficient power to communicate with the base station at an acceptable signal quality.
For these reasons, it is advantageous to limit the number of users which access the system such that loading does not exceed a specified percentage of the pole capacity. One way to limit the loading of the system is to deny access to the system once the loading of the system has reached a predetermined level. For example, if the loading increases above 70% of the pole capacity, it is advantageous to deny requests for additional connection originations and to refrain from accepting hand-off of existing connections.
In order to limit the loading on the reverse link to a specified level, it is necessary to measure the reverse link loading. Reverse link loading of a base station is not solely a function of the number of remote units that are operating within the coverage area of the base station. Reverse link loading is also a function of interference from other sources. The front-end noise of the base station itself is a significant source of interference. In addition, other remote units operating on the same frequency within the coverage area of nearby base stations may contribute significant interference.
One means by which the reverse link loading can be measured is by averaging the measured signal to interference operation point of all active connections within the coverage area. This approach has several drawbacks. The signal to interference operation statistics of the active connections provide an indication of system performance. However, they do not provide any information concerning the amount of interference from remote units located in the coverage area of other base stations. In addition, when a remote unit is in soft hand-off between two or more base stations, it is likely that the actual signal to interference ratio at which the reverse link signal is received at any one base station is significantly beneath the signal to interference ratio set point determined by the system, thus, falsely indicating an extremely high loading level. For these reasons, measuring the average signal to interference operation point of all active connections within a base station does not provide an accurate measure of reverse link loading.
A second and simple means of determining reverse link loading is to simply count the number of active users in the base station. However, because the level of interference from other sources significantly affects loading, it should be clear that the number of users is not necessarily a good indication of reverse link loading. In addition, the effects of soft hand-off greatly decrease the correlation between the number of active users and the actual loading at the base station.
A third means of estimating the reverse link loading is to attempt to derive the reverse link loading based upon an estimate of the forward link loading. However, as noted above, in a typical system the forward and reverse link do not operate at the same frequencies. Therefore, the forward link performance is not perfectly correlated with reverse link performance. For example, the interference from the coverage areas of adjacent base stations can be different on the forward link than on the reverse link. In addition, as noted above, the effects of fading are independent as between the forward and reverse links.
If one of these inaccurate methods of estimating the reverse link loading is used, the system cannot accurately determine whether connection blockage is necessary. If calls are blocked unnecessarily, the capacity of the system is unnecessarily decreased. On the other hand, if the loading is permitted to approach the pole capacity, the probability of dropping a significant number of active connections increases. For this reason, it is important to have an accurate estimation of the reverse link loading.
In his book entitled xe2x80x9cCDMA: Principles of Spread Spectrum Communicationxe2x80x9d (Addison-Wesley Wireless Communications, 1995), Dr. Andrew J. Viterbi defines reverse link loading as a function of the total received power perceived at the base station receiver. The reverse link loading X is directly related to the total power received by the base station according to the following formula:                                           P            a                                P            n                          =                  1                      1            -            X                                              (        1        )            
where:
Pa is the actual power received at the base station;
Pn is the power received at no external loading (e.g. the power due to the thermal noise floor of the base station); and
X is the reverse link loading in terms of the ratio of actual loading to pole capacity.
Or equivalently, expressed in terms of X, Equation 1 takes on the following expression:                     X        =                                            P              a                        -                          P              n                                            P            a                                              (        2        )            
For example, this formula states that at 50% loading (X=0.5), the total power received at the base station is twice that which is received at no loading.
Given the relationship shown in Equation 1, current base station loading X can be determined based upon a known no load power level and an actual measurement of the total power received at the base station. Note that the actual power measurement should be filtered with an appropriate time constant in view of the time constant at which the power control operation varies the transmit power of the remote unit. In addition, if the reverse link operates at variable data rates which result in gated transmissions from the remote units, the actual power measurement should be filtered to average the effects of the gated transmissions on the instantaneous power measurement.
The dynamic range of the relative power measurement (Pa/Pn) is not large in a typical system. For example, as the loading X increases from 0 to 90% of the pole capacity, the ratio of (PJPn) increases from 0 to 10 decibels (dB). Typically, base station loading X is limited to about 60-75% of the pole capacity. As X increases from 0.6 to 0.75, the ratio of (Pa/Pn) increases from about 4 to about 6 dB. Therefore, to accurately limit the loading of the reverse link, the ratio of (Pa/Pn) should be measured with less than 1 dB of error in order to avoid over- or under-estimation of the loading.
While this approach appears to be straight-forward, in reality, it is difficult to consistently achieve the required accuracy of the relative power measurements. The difficulty stems from the complexity of making absolute power measurement of both Pn and Pa, within required accuracy of a few dB. Furthermore, even if an accurate absolute measurement of the noise floor could be made at one time, the noise floor is sensitive to gain and noise figure variations due to temperature, aging and other phenomenon and, hence, the noise floor power level changes as a function of time. Without a means of accurate measurement, any admission control algorithm based upon Equation 2 will likely block connections when no blocking is necessary or admit connections in the face of potentially unsteady system behavior.
For these reasons, there has been a long felt need in the industry for a method and apparatus for accurately estimating the reverse link loading of a system.
In order to determine a current level of loading, a system enters a period of silence, in which reverse link transmissions are inhibited.
In one embodiment of the invention, a remote test unit generates a reverse link signal during the period of silence. A base station demodulates the reverse link signal and generates a series of closed loop power control commands for the remote unit. The remote unit responds to the power control commands by adjusting the level at which it transmits the reverse link signal. As the system operating point changes in response to the new operating conditions, the series of commands are accumulated to determine a transmit gain adjustment value corresponding to the period of silence, TGA(0). Once normal system operation is resumed, the base station demodulates the reverse link signal from the remote test unit and generates a series of power control commands for the remote unit. As the system operating point changes in response to the normal operating conditions once again, the series of power control commands are accumulated to determine a transmit gain adjustment value for the current system loading, TGA (t). Using TGA(0) and TGA(t), the system loading is determined.
In another embodiment of the invention, a known low energy deterministic signal is injected into the base station receiver prior to signal-to-strength detection circuit. During the period of silence, a signal-to-noise ratio due to this SS signal is measured, and used as a calibrated reference for the noise floor Pn. Once normal system operation is resumed, the signal-to-noise ratio due to this signal, now degraded by other reverse link signals, is measured, and used as a measure of the power level Pn. The load is then calculated using these measurements. At the next period of silence a new calibrated reference measurement reflecting the noise floor Pn is taken. Once normal system operation is resumed, and measurements of the reverse link signal Pa are taken, the load is calculated using this new Pn .
In yet another embodiment of the invention, during the period of silence, the base station receiver makes an observation, which provides a calibrated reference for the noise floor Pn. Once normal system operation is resumed, the base station receiver takes measurements of the reverse link signal Pn. The load is then calculated by means of Equation 2. At the next period of silence a new calibrated reference measurement of the noise floor Pn is taken. Once normal system operation is resumed, and measurements of the reverse link signal Pa are taken, Equation 2 is again utilized, using this new Pn . Thus, this approach eliminates the need for an absolute power measurement, and accounts for any drift caused with physical phenomena.